Most HSLA steels are produced in conventional processes where molten steel from a basic oxygen furnace (BOF) or an electric arc furnace (EAF) is cast, cooled, reheated and reduced in thickness while still hot in a rolling mill. The rolling mill reduces the thickness of the slab to produce thin gauge steel sheet or strip material having high strength characteristics. Some HSLA steels are produced by modern thin-slab or medium-slab casting processes in which slabs of steel, still hot from the caster, are transferred directly to a reheating or equalizing furnace prior to thickness reduction in the hot rolling mill.
HSLA steel products are commonly used for automotive and other applications where high strength and reduced weight are required. Such applications also require material having good formability to allow it to be shaped into parts.
Due to the steel microstructure and metallurgical transformations taking place in the material during hot rolling, reducing the gauge of the material also causes the material to become harder. As the hardness increases, further thickness reduction by rolling becomes more difficult, and the rolling mill must operate with increasing power levels to reduce the material thickness to the desired level at a particular width. Due to the high power required to reduce the thickness, higher strength HSLA sheet or strip material, typically having a strength above about 350 MPa, is only available in limited widths.
As the strength of the material is increased through rolling, the subsequent formability of the material in service is reduced. This makes shaping of the material more difficult. Thus, rolling the HSLA material to light gauges interferes with the ability to shape the material, limiting its utility for many applications requiring high strength, light weight and good formability, such as automotive applications.
Therefore, there is a need for HSLA steel products having high strength, thin gauge and acceptable formability.